In conventional air-conditioning apparatuses such as a multi-air-conditioning apparatus for a building, a refrigerant is circulated between an outdoor unit, which is a heat source unit disposed, for example, outside a structure, and indoor units disposed in rooms in the structure. A conditioned space is cooled or heated by heated or cooled air, from which a refrigerant has released or absorbed heat. Regarding the refrigerant used for such an air-conditioning apparatus, a hydrofluorocarbon (HFC) refrigerant, for example, is typically used. An air-conditioning apparatus using a natural refrigerant, such as carbon dioxide (CO2), has also been proposed.
Furthermore, in an air-conditioning apparatus called a chiller, cooling energy or heating energy is generated in a heat source unit disposed outside a structure. Water, antifreeze, or the like is heated or cooled by a heat exchanger disposed in an outdoor unit and is carried to an indoor unit, such as a fan coil unit or a panel heater, to perform heating or cooling (see Patent Literature 1, for example).
Moreover, there is an air-conditioning apparatus called an exhaust heat recovery chiller that connects a heat source unit to each indoor unit with four water pipes arranged therebetween, supplies cooled and heated water or the like simultaneously, and allows the cooling and heating in the indoor units to be selected freely (see Patent Literature 2, for example).
In addition, there is an air-conditioning apparatus in which a heat exchanger for a primary refrigerant and a secondary refrigerant is disposed near each indoor unit so that the secondary refrigerant is carried to the indoor unit (see Patent Literature 3, for example).
Furthermore, there is an air-conditioning apparatus that connects an outdoor unit to each branch unit including a heat exchanger with two pipes so that a secondary refrigerant is carried to an indoor unit (see Patent Literature 4, for example).
Additionally, there is an air-conditioning apparatus such as a multi-air-conditioning apparatus for a building that, while circulating a heat medium such as water to indoor units, reduces conveyance power of the heat medium by circulating a refrigerant from an outdoor unit to a relay unit and by circulating the heat medium, such as water, from the relay unit to the indoor units (see Patent Literature 5, for example).